1. Field of the Invention
The present invention relates generally, to the field of metallurgy and more specifically to a method of forming a substantially uniform coating of a stoichiometric, titanium silicides over the surface of titanium and base alloys thereof.
2. Description of the Prior Art
Titanium is frequently used to fabricate structural or load-bearing members. Because of its relatively low density (about 0.16 lb. per cubic inch compared to about 0.28 for steel) it is often used in applications which require high strength, but where weight considerations are important, such as in the construction of aircraft. Because titanium is substantially nontoxic to humans and animals, it has also been extensively used in the construction of biomedical implants.
Titanium and titanium alloys do not exhibit good, high temperature oxidation resistance. It is well known that metallic titanium oxidizes very readily, even at room temperature at room temperature, metallic titanium quickly forms a thin oxide surface coating that is highly resistant to the diffusion of additional oxygen. The thin oxide surface coating is also very resistant to chemical attack. Unfortunately, at elevated temperatures the underlying metal will continue to rapidly oxidize. For this reason, titanium and its base alloys have generally been employed only in air or combustion gas environments where service temperatures are less than about 500.degree. C.
Numerous attempts have been made to improve the oxidation and corrosion resistance of the titanium and titanium alloys and other metals. During the 1950's and '60's many methods were directed at forming a stoichiometric, metal silicide coating on substrates fabricated from the metals and base alloys thereof . The term "siliciding" will be used herein to broadly designate any process which accomplishes this result. These prior art siliciding methods generally employed the diffusion of elemental silicon into the substrate, at its surface. Specific examples of these prior art methods ar described below.
As used herein in connection with a metal or other element, the term "base alloy" means an alloy which is comprised of at least 50 weight percent of the designated metal or element. Consistent with convention (which is followed hereinafter unless otherwise indicated), such alloys are generally written in a form which does not specifically include the term "weight percent" in connection with the base metal or alloying constituents. As an example of this convention, the familiar aircraft alloy which comprises a titanium metal base, 6 weight percent aluminum and 4 weight percent vanadium is simply written Ti-6A1-4V;. Thus, Ti-6A1-4V is referred herein as a base alloy of titanium or a titanium-based alloy.
Also as used herein, the term "stoichiometric metal silicide," or "intermetallic silicide" means stoichiometric intermetallic compounds which exist in a binary alloy system between a particular metal and silicon. The intermetallic silicides (sometimes also referred to hereinafter simply as "metal silicides" or "silicides") exist as distinct crystalline phases, with no more than a narrow range of compositions about the stoichiometric proportion. A given metal-silicon alloy system may include several metal silicides of different stoichiometric relation. It will be understood by those skilled in the art that metal silicides may also exist in higher (ternary, quaternary etc.) alloy systems, so long as the metal and silicon are present in the required proportions and the crystal lattice assumes the requisite phase structure. As will be illustrated below in connection with the present invention, a titanium silicide coating (including a plurality of silicides) may be formed on a substrate fabricated from Ti-6A1-4V alloy. While the titanium silicide coating is comprised substantially of titanium silicides, the coating may also include vanadium and aluminum.
Several prior art attempts at siliciding the metals are reported in Coating of High-Temperature Materials (Samsonov, G. V., et al.; Hausner, H. ed; Plenum Press, New York 1966). A good deal of the work was carried out in the Soviet Union and involves the use of silicon tetrachloride, in a gaseous phase, as the silicon metal source. According to the siliciding theory, the gaseous silicon tetrachloride is reduced by hydrogen, which in turn causes the deposit of elemental silicon on the surface of the metal substrate. It is believed that the "metallic" silicon which is so-deposited, thereafter diffuses into the metal substrate and forms the desired metal silicide or silicides at the surface of the substrate. The process was reportedly carried out at temperatures between about 800.degree. C. and 1200.degree. C. on titanium, tantalum and molybdenum substrates. The starting components for generating the silicon tetrachloride and hydrogen were reported to include silicon powder mixed with three percent ammonium chloride.
The above process has several drawbacks, the most important of which is the presence of hydrogen. It is well known that at the reported temperatures, many metals, and particularly titanium, exhibit an extremely high solid solubility of hydrogen. It is also well known that very low concentrations of dissolved hydrogen can have a very detrimental effect on the mechanical properties of metals. In titanium, concentrations as low as 200-300 parts per million can induce brittleness and substantially reduce fatigue life. Thus, while the hydrogen reduction of silicon tetrachloride can provide a metal silicide coating on a metal substrate, the mechanical properties of the substrate may be severely affected.
Other prior art methods for siliciding metals have included "pack siliciding." In pack siliciding a metal substrate is surrounded by silicon powder (mixed with an inert separating compound) in a closed container. The entire container and its contents are then heated to and soaked at an elevated temperature so that the silicon diffuses into the metal substrate under solid state conditions. This method suffers from the drawback that the substrate must be subjected to diffusion temperatures for very long periods of time in order to form a silicide coating of appreciable thickness. Such a long term thermal excursion can adversely affect the microstructure of the metal and hence, its mechanical properties.
Furthermore, a dense silicide coating of substantially uniform thickness is not produced by solid state diffusion from a powder. The true area of contact between the surface of a substrate and a powder covering the substrate, is substantially less than the measured surface of that substrate. Because diffusion can occur only at the points of contact between the metal substrate and the silicon powder, the diffusion rate, as measured over the entire surface area of the substrate, is quite slow. In addition, as silicon diffuses into the metal substrate, the metal from the substrate diffuses into the silicon powder. This process produces a very porous silicide layer.
A general method of providing a coating on metals and alloys by diffusion is disclosed in French Patent No. 1,312,819. In this process a small amount of a coating material (generally between 10 and 1000 parts per million) in an alkali metal bath is used to coat the metal substrate, the only example of a silicide coating is molybdenum silicide formed on a molybdenum substrate.
The counterpart of the French Patent was U.S. Ser. No. 85,457 filed Jan. 10, 1961 and subsequently abandoned in favor of two continuation-in-part applications which matured into U.S. Pat. Nos. 3,192,065 and 3,220,876. In U.S. Pat. No. 3,192,065 the inventors disclosed that the molybdenum silicide formed by the process disclosed in the earlier process was of irregular thickness and varied performance lifetimes. They taught that it was necessary to dissolve at least one additive from the group carbon and tin in the bath.
French Patent No. 1,388,934 discloses the use of an alkaline earth metal such as calcium as a transfer agent to give diffusion alloy coatings on refractory metals. The diffusing elements are usually mixtures of aluminum and silicon. However, a 50% solution of silicon in calcium was used to give a multilayer coating on niobium.
Another disclosure of the use of a mixture of calcium and silicon to form a silicide diffusion coating is Australian Patent No. 290,492. The preferred amounts of silicon in the mixture are from 1% to 10%, and the mixture is used to coat steel.
It is clear that none of the references disclose a process directed to the rapid formation of a dense silicide coating of uniform thickness on titanium and its base alloys. We have discovered, surprisingly, that if titanium or its base alloys are contacted at the proper temperature with a molten alloy of lithium and silicon, containing at least about sixty weight percent of silicon, a dense silicide coating of uniform thickness is readily formed on the titanium or its base alloys.